Tomato Problems: Bacterial Diseases in Tomatoes by Mary Small

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Moist weather in spring and summer can contribute to the development of bacterial diseases on tomatoes, just like it does for fireblight. The two diseases most often seen in moist years are bacterial speck (Pseudomonas syringae pv.tomato) and bacterial spot (Xanthomonas campestris pv. vesicatoria).

Leaf symptoms look the same for both diseases. Small water-soaked spots form and grow to about 1/8” in size with yellow halos. The centers are light brown and often tear; yellow halos are common. On more mature plants, infections are concentrated on the older foliage. Spots may also appear on the fruit pedicels.

You can often tell the two diseases apart by checking the immature fruit. Bacterial spot lesions are small, water-soaked spots that grow to about 1⁄4” and become raised. Bacterial speck lesions are black and sunken, about 1/16” in size. Bacterial speck is more likely to develop in cool moist conditions (64 to 75 degrees F) and bacterial spot is more likely to develop under warm moist conditions (75 to 90 degrees F). Hard rains and mechanical injury from hail, blowing soil particles or wind increase the severity of the diseases.

Preventive measures are best in the course of both these diseases. Crop rotation and garden sanitation will help as the diseases can overwinter on plant debris. Unfortunately, the disease can also enter the garden on infested seed or transplants. Don’t handle or work around the plants when wet. Copper containing fungicides can be applied at the first sighting of leaf spots and can be repeated every 7 to 10 days, but may not completely stop the disease under favorable weather conditions.

For more information about tomato problems in the home garden, check out this CSU fact sheet: Recognizing Tomato Problems.

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